This invention relates to a fuel injection system for a mixture compressing spark ignition internal combustion engine having means for injection of fuel into the air intake of the engine, the air intake including an adjustable throttle valve and an air flow measuring element which is movable against a restoring force in accordance with the quantity of air flowing through the intake the measuring element being arranged to actuate the control element of a fuel metering valve for dispensing to one or more injection jets a quantity of fuel related to the quantity of air. In such systems a situation may arise at temperatures below freezing point whereby, by virtue of the water contained in the fuel, the control piston of the metering valve freezes solid, so that a cold start is made considerably more difficult and in some cases even impossible. As a result of the low negative pressures obtaining in the air intake during the starting operation, the measuring element does not generate sufficient force to free the control piston.